A285378 - OEIS

%I #14 Jun 08 2024 15:41:34

%S 1,2,4,12,15,21,30,78,93,160,244,490,574,1196,1275,1875,2590,5732,

%T 7344,9728,11918,13840,33945,100075,110922,117991,142440

%N Numbers k such that (14*10^k + 61)/3 is prime.

%C For k > 1, numbers k such that the digit 4 followed by k-2 occurrences of the digit 6 followed by the digits 87 is prime (see Example section).

%C a(28) > 2*10^5.

%H Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>.

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 46w87</a>.

%e 4 is in this sequence because (14*10^4 + 61)/3 = 46687 is prime.

%e Initial terms and associated primes:

%e a(1) = 1, 67;

%e a(2) = 2, 487;

%e a(3) = 4, 46687;

%e a(4) = 12, 4666666666687;

%e a(5) = 15, 4666666666666687; etc.

%t Select[Range[0, 100000], PrimeQ[(14*10^# + 61)/3] &]

%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.

%K nonn,more,hard

%O 1,2

%A _Robert Price_, Apr 17 2017

%E a(24)-a(27) from _Robert Price_, Dec 06 2018